The use of high energy electron beams to effect welding of metal is well known. The welding process must take place in a high vacuum in order to generate an electron beam of high kinetic energy. The high energy beam impinges on the joint between the metals being welded together, the energy of the beam causing the metals to melt and fuse together.
Because the welding process must be carried out in a high vacuum, some optical arrangement is generally required to permit the operator to view the work piece within the vacuum chamber. Whatever optical arrangement is used, by transmitting light from the impact area, is necessarily exposed to other wave lengths of energy from the impact area, such as infrared and X-ray radiations, but also is exposed to atomic vapor of the materials being welded which deposit on surfaces of the optical system and cause it to become opaque. These deposits must be removed frequently, which requires opening the chamber to atmosphere and substantially reducing the operating time and efficiency of the welding device.
In U.S. Pat. No. 3,156,810 there in described a viewing system for an electron beam welder in which a transparent strip of electrically conductive material is movable across the surface of the optical system exposed to the metallic vapors. This strip can be advanced as vapor deposits accumulate on the surface of the strip so as to clear the optical system of deposited materials.
However, the transparent strip of plastic materials is exposed to infrared and X-ray radiations generated by the impact of the electron beam with the work piece, producing destructive heating and discoloration of the plastic material and possible damage to the glass of the reflecting element. Also, static charges tend to build up on the plastic strip which can influence the focusing of the electron beam. The above-identified patent suggests using a plastic material having a conductive surface to dissipate the electric charge and to use a metal mirror rather than a glass-reflecting element to lessen the effect of X-rays. However, the rate of clouding and discoloration due to radiations from the impact area greatly increases the frequency at which the strip must be advanced and replaced, and the addition of the conductive coating on the plastic strip increases its cost.